Semiconductor light generating device

ABSTRACT

The semiconductor light generating device comprises a light generating region  3,  a first Al X1 Ga 1-X1 N semiconductor (0≦X1≦1) layer  5  and a second Al X2 Ga 1-X2 N semiconductor (0≦X2≦1) layer  7.  In this semiconductor light generating device, the light generating region  3  is made of III-nitride semiconductor, and includes a InAlGaN semiconductor layer. The first Al X1 Ga 1-X1 N semiconductor (0≦X1≦1) layer  5  is doped with a p-type dopant, such as magnesium, and is provided on the light generating region  3.  The second Al X2 Ga 1-X2 N semiconductor layer  7  has a p-type concentration smaller than the first Al X1 Ga 1-X1 N semiconductor layer  5.  The second Al X2 Ga 1-X2 N semiconductor (0≦X2≦1) layer  7  is provided between the light generating region  3  and the first Al X1 Ga 1-X1 N semiconductor layer  5.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light generating deviceand a method of the same.

2. Related Background of the Invention

Publication 1 (Japanese Patent Application Laid-Open No. 2001-237455)discloses an InAlGaN (indium aluminum gallium nitride) semiconductormaterial and an ultraviolet light generating device which uses theInAlGaN semiconductor and emits light in an ultraviolet wavelengthregion. This InAlGaN semiconductor light generating device can emitultraviolet light at room temperature.

Publication 2 (Japanese Patent Application Laid-Open No. 2001-119068)discloses an ultraviolet light generating device. This ultraviolet lightgenerating device has an undoped AlGaN cladding layer provided betweenan AlGaN active layer and a p-type AlGaN cladding layers and thisundoped AlGaN cladding layer is not less than 10 nanometers. The undopedAlGaN cladding layer prevents electrons in the active layer fromtransiting to magnesium accepter levels in the p-type AlGaN claddinglayer. Using the undoped AlGaN cladding layer, the light generatingdevice can use an sapphire substrate which is not expensive. In order toavoid the transition of electrons in the active layer to the Mg-dopedAlGaN layer, the active layer has to be separated from the Mg-dopedAlGaN layer and an undoped layer is needed between the active layer andthe Mg-doped AlGaN layer.

SUMMARY OF THE INVENTION

The bandgap of the InAlGaN semiconductor can be widely varied dependingon its composition. A semiconductor light generating device havingInAlGaN light generating region can generate light of ultravioletwavelength or more longer. In order to confine carriers to the lightgenerating region in the semiconductor light generating device, acarrier blocking layer is formed therein and the carrier blocking layeris made of AlGaN semiconductor capable of providing a potential barriergreater than that of InAlGaN semiconductor.

The inventors' experiment has revealed that the light generatingefficiency of the semiconductor light generating device is decreased dueto p-type dopant atoms diffusing to a InAlGaN semiconductor layer in theactive region from a p-type semiconductor region adjacent thereto

It is an object of the present invention to provide a semiconductorlight generating device the light generating efficiency of which isenhanced, and a method of making the same.

One aspect of the present invention is a method of making asemiconductor light generating device. The method comprises the stepsof: (a) forming an undoped InAlGaN semiconductor film for an activeregion; (b) forming an undoped Al_(X2)Ga_(1-X2)N semiconductor film(0≦X2≦1) on the InAlGaN semiconductor film; and (c) forming a p-typeAl_(X1)Ga_(1-X1)N semiconductor film (0≦X1≦1) and one or more galliumnitride based semiconductor films on the undoped Al_(X2)Ga_(1-X2)Nsemiconductor film and causing p-type dopant atoms in the p-typeAl_(X1)Ga_(1-X1)N semiconductor film to diffuse to the undopedAl_(X2)Ga_(1-X2)N semiconductor film.

In the above method, aluminum composition of the Al_(X2)Ga_(1-X2)Nsemiconductor film is greater than that of the p-type Al_(X1)Ga_(1-X1)Nsemiconductor film (0≦X1≦1).

According to another aspect of the present invention, a semiconductorlight generating device comprises (a) a light generating regionincluding a InAlGaN semiconductor layer; (b) a first Al_(X1)Ga_(1-X1)Nsemiconductor layer (0≦X1≦1) doped with p-type dopant; and (c) a secondAl_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1) having a p-type dopantconcentration smaller than that of the first Al_(X1)Ga_(1-X1)Nsemiconductor layer, the second Al_(X2)Ga_(1-X2)N semiconductor layerbeing provided between the InAlGaN semiconductor layer and the firstAl_(X1)Ga_(1-X1)N semiconductor layer.

In the semiconductor light generating device, the secondAl_(X2)Ga_(1-X2)N semiconductor layer includes a region having a p-typeconcentration of 3×10¹⁸ cm⁻³ or lower, and this region is 1 nanometer ormore.

According to still another aspect of the present invention, asemiconductor light generating device comprises: (a) a light generatingregion including a InAlGaN semiconductor layer; (b) a firstAl_(X1)Ga_(1-X1)N semiconductor layer (0≦X1≦1) doped with p-type dopant;and (c) a second Al_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1) havingan aluminum composition greater than that of the first Al_(X1)Ga_(1-X1)Nsemiconductor layer, the second Al_(X2)Ga_(1-X2)N semiconductor layerbeing provided between the InAlGaN semiconductor layer and the firstAl_(X1)Ga_(1-X1)N semiconductor layer, and the p-type dopantconcentration of the second Al_(X2)Ga_(1-X2)N semiconductor layer beingsmaller than that of the first Al_(X1)Ga_(1-X1)N semiconductor layer.

In the semiconductor light generating device, thickness of the secondAl_(X2)Ga_(1-X2)N semiconductor layer is greater than or equal to fivenanometers.

In the semiconductor light generating device, the firstAl_(X1)Ga_(1-X1)N semiconductor layer is doped with magnesium of p-typedopant, and the first Al_(X1)Ga_(1-X1)N semiconductor layer includes aregion having a magnesium concentration of 1×10¹⁹ cm⁻³ or higher.

In the semiconductor light generating device, the secondAl_(X2)Ga_(1-X2)N semiconductor layer is doped with magnesium dopant,and a p-type dopant concentration at an interface between the lightgenerating region and the second Al_(X2)Ga_(1-X2)N semiconductor layeris less than 3×10¹⁸ cm⁻³ or lower.

In the semiconductor light generating device, a maximum concentration ofp-type dopant in the first Al_(X1)Ga_(1-X1)N semiconductor layer is notless than 1×10²⁰ cm⁻³, and a p-type dopant concentration at an interfacebetween the light generating region and the second Al_(X2)Ga_(1-X2)Nsemiconductor layer is not greater than 3×10¹⁸ cm⁻³.

The semiconductor light generating device, further comprises asupporting body of III-group nitride, the light generating region, thefirst Al_(X1)Ga_(1-X1)N semiconductor layer and the secondAl_(X2)Ga_(1-X2)N semiconductor layer being provided on the supportingbody.

In the above semiconductor light generating device, the secondAl_(X2)Ga_(1-X2)N semiconductor layer is formed as an undoped layer.

In the semiconductor light generating device, the secondAl_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1) has an aluminumcomposition greater than that of the first Al_(X1)Ga_(1-X1)Nsemiconductor layer.

In the semiconductor light generating device, the light generatingregion further includes one or more InAlGaN semiconductor barrier layersand one or more InAlGaN semiconductor well layers.

The light generating semiconductor device includes a lightly dopedsemiconductor region on the light generating region and a semiconductorregion of a low resistance in the second AlGaN semiconductor layerformed by doping it with dopant atoms diffusing from the first AlGaNsemiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described object and other objects, features, and advantagesof the present invention will become apparent more easily in thedetailed description of the preferred embodiments of the presentinvention which will be described below with reference to theaccompanying drawings.

FIG. 1 is a perspective view showing a semiconductor light generatingdevice according to the present embodiment;

FIG. 2 is a view showing a modified semiconductor light generatingdevice;

FIG. 3 is a view showing a modified semiconductor light generatingdevice;

FIG. 4 is a graph showing a optical spectrum of a light emitting diodeshown in an example;

FIG. 5 is a graph showing a optical power of a light emitting diodeshown in an example;

FIG. 6 is a graph showing a secondary ion mass spectroscopy (SIMS)analysis of light emitting diode A;

FIG. 7 is a graph showing a secondary ion mass spectroscopy (SIMS)analysis of light emitting diode B;

FIG. 8 is a graph showing optical powers of the light emitting diodes Aand B;

FIG. 9 is a graph showing optical powers of light emitting diodes;

FIG. 10 is views explaining steps in a method of making a semiconductorlight generating device;

FIG. 11 is views explaining steps in a method of making a semiconductorlight generating device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The teachings of the present invention will readily be understood inview of the following detailed description with reference to theaccompanying drawings illustrated by way of example. Referring to theaccompanying drawings, embodiments of the semiconductor light generatingdevice according to the present invention will now be explained. Whenpossible, parts identical to each other will be referred to withnumerals identical to each other.

First Embodiment

FIG. 1 shows a semiconductor light generating device according to thefirst embodiment. This semiconductor light generating device has astructure preferable to a surface emitting device such as a lightemitting diode.

The semiconductor light generating device comprises a light generatingregion 3, a first Al_(X1)Ga_(1-X1)N semiconductor (0≦X1≦1) layer 5 and asecond Al_(X2)Ga_(1-X2)N semiconductor (0≦X2≦1) layer 7. In thissemiconductor light generating device 1, the light generating region 3is made of III-group nitride semiconductor, and includes a InAlGaNsemiconductor layer. The first Al_(X1)Ga_(1-X1)N semiconductor (0≦X1≦1)layer 5 is doped with a p-type dopant, such as magnesium, and isprovided on the light generating region 3. The second Al_(X2)Ga_(1-X2)Nsemiconductor (0≦X2≦1) layer 7 is provided between the light generatingregion 3 and the first Al_(X1)Ga_(1-X1)N semiconductor layer 5. Thesecond Al_(X2)Ga_(1-X2)N semiconductor layer 7 has a p-type dopantconcentration smaller than that of the first Al_(X1)Ga_(1-X1)Nsemiconductor layer 5.

Since the second Al_(X2)Ga_(1-X2)N semiconductor layer 7 separates thelight generating region 3 from the first Al_(X1)Ga_(1-X1)N semiconductorlayer 5, p-type dopant atoms in the first Al_(X1)Ga_(1-X1)Nsemiconductor layer 5 diffuse into the second Al_(X2)Ga_(1-X2)Nsemiconductor layer 7 in the fabrication process. The secondAl_(X2)Ga_(1-X2)N semiconductor layer 7 lowers the number of p-typedopant atoms which diffuse from the first Al_(X1)Ga_(1-X1)Nsemiconductor layer 5 to the light generating region 3, therebyenhancing the light generating efficiency of the semiconductor lightgenerating device.

There are a number of pits on the surface of InAlGaN semiconductorlayer. These pits are located in the high threading dislocation region.If an AlGaN semiconductor layer doped relatively heavily is deposited onthe InAlGaN semiconductor layer, p-type dopant atoms in this p-typeAlGaN layer diffuse to the light generating region by the aid of thepits. On the other side, since an AlGaN semiconductor layer having arelatively low carriers concentration is provided on the InAlGaNsemiconductor layer instead of not a heavily doped AlGaN semiconductorlayer, the number of the atoms diffusing to the InAlGaN layer isreduced. A semiconductor region having a low carrier concentration isformed just on the second Al_(X2)Ga_(1-X2)N semiconductor layer 7 andthe light generating region includes a semiconductor region theresistance of which is lowered by dopant atoms from the firstAl_(X1)Ga_(1-X1)N semiconductor layer 5.

The light generating device 1 includes a third Al_(X3)Ga_(1-X3)Nsemiconductor (0≦X3≦1) layer 9. The third Al_(X3)Ga_(1-X3)Nsemiconductor layer 9 is doped with n-type dopant, such as silicon. Inan example, the light generating region 3 is formed on the thirdAl_(X3)Ga_(1-X3)N semiconductor layer 9. The light generating region 3is provided between the first Al_(X1)Ga_(1-X1)N semiconductor layer 5and the third Al_(X3)Ga_(1-X3)N semiconductor layer 9.

The third Al_(X3)Ga_(1-X3)N semiconductor layer 9 supplies electrons tothe light generating region 3 and the first Al_(X1)Ga_(1-X1)Nsemiconductor layer 5 supplies holes to the light generating region 3through the second Al_(X2)Ga_(1-X2)N semiconductor layer 7. Thesecarriers (electrons and holes) are confined into the light generatingregion 3 due to the two AlGaN semiconductor layers and the recombinationof the carriers generates light in the light generating region.

The light generating device 1 includes a supporting body 11. Thesupporting body 11 mounts the first to third AlGaN layers 5, 7, 9 andthe light generating region 3 on its primary surface. In a preferredexample, the supporting body 11 may be a III-group nitride supportingbody 13, and the supporting body 11 can includes a buffer layer 15provided on the primary surface of the III-group nitride supporting body13. The III-group nitride supporting body 13 may be made of galliumnitride (GaN), aluminum nitride (AlN) and so on, and the following canbe used as material for the supporting body: silicon carbide (SiC) andzirconium di-boride (ZrB₂).

In a preferred example, the supporting body 11 may be made of galliumnitride. This gallium nitride supporting body has a low density ofthreading dislocations which are formed in the light generating region 3and which act as non-radiative recombination centers.

The light generating device 1 includes a contact layer 17. The contactlayer 17 is provided on the AlGaN semiconductor layer 5. Thesemiconductor light generating device 1 further includes a cathodeelectrode 21 provide on the backside of the supporting body and an anodeelectrode 23 provided on the contact layer 17.

Area (A) in FIG. 2 shows a modified semiconductor light generatingdevice. Area (B) in FIG. 2 shows a p-type dopant profile. The modifiedsemiconductor light generating device 1 a has a light generating region3 a in place of the light generating region 3. The light generatingregion 3 a has one or more well layers (for example, well layer 25 a, 25b, 25 c) and a plurality of barriers layers (for example, barrier layers27 a, 27 b, 27 c, 27 d). The well layer 25 a, 25 b, 25 c and thebarriers layers 27 a, 27 b, 27 c, 27 d are arranged alternately. Each ofthe barriers layers 27 a, 27 b, 27 c, 27 d has a potential barrier tothe well layer 25 a, 25 b, 25 c. The light generating region 3 a arelocated between the two AlGaN semiconductor regions and these AlGaNsemiconductor regions have potential barriers to the barrier layers 27a, 27 e, respectively. The light generating region 3 a has a MQWstructure, but subsequent explanation may apply to single quantum well(SQW) structures as well.

Referring to area (B) in FIG. 2, a curve indicating a p-type dopantconcentration is shown as a function of the coordinate. In thesubsequent explanation, a p-type dopant profile is explained withreference to AlGaN layers each containing magnesium atoms as a p-typedopant. Profile P1 indicates the magnesium dopant concentration andmonotonically decreases from the first AlGaN semiconductor layer to thelight generating region 3.

In a preferred example, the magnesium concentration of the first AlGaNsemiconductor layer 5 is greater than 1×10¹⁹ cm⁻³ (indicated by ArrowN1). This heavy doping in the semiconductor light generating device laprevents the resistance of the first Al_(X1)Ga_(1-X1)N semiconductorlayer 5 from increasing even when a portion of the magnesium atomsdiffuse from the first AlGaN semiconductor layer 5 to the second AlGaNsemiconductor layer 7.

In a preferred example for the semiconductor light generating device 1a, the magnesium concentration at the interface (indicated by Arrow J1)between the light generating 3 a and the second AlGaN semiconductorlayer 7 is smaller than 3×10¹⁸ cm⁻³ (indicated by Arrow N2). This secondAlGaN semiconductor layer 7 is effective in avoiding the increase of thep-type dopant concentration in the light generating region 3.

In a preferred example, the magnesium concentration at the interface(indicated by Arrow J2) between the first AlGaN semiconductor layer 5and the second AlGaN semiconductor layer 7 is larger than 1×10¹⁹ cm⁻³(indicated by Arrow N3). This first AlGaN semiconductor layer 5 preventsthe p-type dopant concentration in the light generating region 3 fromincreasing to such a extent that the light generating efficiency of thesemiconductor light generating device 1 a is deteriorated.

Additionally, in a preferred example, the maximum magnesiumconcentration of the first AlGaN semiconductor layer 5 is not less than1×10²⁰ cm⁻³ (indicated by Arrow N4). This light generating device 1 a,the resistance of the first AlGaN semiconductor layer 5 still remainslow.

The light generating device 1 a includes a region which is providedbetween the first AlGaN semiconductor layer 5 and the light generatingregion 3 and which has a p-type dopant concentration not greater than3×10¹⁸ cm⁻³. The thickness D is not less than one nanometer. If thethickness falls within this range, the light generating region isseparated sufficiently from the first AlGaN semiconductor layer 5 tolower the p-type dopant concentration in the light generating region.

In a preferred example, a part contributing to the light generation inthe light generating region has a magnesium concentration of 1×10¹⁷ cm⁻³or less.

In a preferred example, the thickness of the second AlGaN semiconductorlayer 7 is not less than one nanometer in the semiconductor lightgenerating device 1 a. If this thickness falls within the range, thelight generating region 3 a is separated sufficiently from the firstAlGaN semiconductor layer 5. The layered AlGaN semiconductor regionadjacent to the light generating region 3 a has a magnesiumconcentration of not more than 1×10¹⁷ cm⁻³. If this concentration fallswithin the range, the magnesium concentration in the light generatingregion 3 a becomes sufficiently low.

In a preferred example, the thickness of the second Al_(X2)Ga_(1-X2)Nsemiconductor layer is not more than 50 nanometers. If this thicknessfalls within the above range, the efficiency of the carrier injection tolight generating region 3 a is in an acceptable range.

Since the second Al_(X2)Ga_(1-X2)N semiconductor layer is formed as anundoped semiconductor layer, a p-type dopant concentration at theinterface between the second Al_(X2)Ga_(1-X2)N semiconductor layer andthe light generating region is smaller than an n-type dopantconcentration at the interface between the third Al_(X3)Ga_(1-X3)Nsemiconductor layer and the light generating region.

Area (A) in FIG. 3 shows a modified semiconductor light generatingdevice. Area (B) in FIG. 3 is a diagram showing potential barrier inthis semiconductor light generating device.

The semiconductor light generating device 1 b comprises a lightgenerating region 3 b, a first Al_(X1)Ga_(1-X1)N semiconductor layer(0≦X1≦1) 31 and a second Al_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1)33. In this semiconductor light generating device 1 b, the lightgenerating region 3 b is made of III-group nitride semiconductor andincludes an InAlGaN semiconductor layer. The second Al_(X2)Ga_(1-X2)Nsemiconductor layer 33 has a p-type concentration smaller than that ofthe first Al_(X1)Ga_(1-X1)N semiconductor layer 31. The secondAl_(X2)Ga_(1-X2)N semiconductor layer 33 is provided between the lightgenerating region 3 b and the first Al_(X1)Ga_(1-X1)N semiconductorlayer 31.

Since the second Al_(X2)Ga_(1-X2)N semiconductor layer 33 is providedbetween the InAlGaN semiconductor layer and the first Al_(X1)Ga_(1-X1)Nsemiconductor layer 31, the p-type dopant atoms in the firstAl_(X1)Ga_(1-X1)N semiconductor layer 31 diffuse in the manufacturingprocess. The second Al_(X2)Ga_(1-X2)N semiconductor layer 33 iseffective in reducing the number of p-type dopant atoms diffusing fromthe first Al_(X1)Ga_(1-X1)N semiconductor layer 31 to the lightgenerating device 3 b. Therefore, the light generating efficiency of thelight generating device 1 b is improved.

If the aluminum composition in AlGaN semiconductor is increased, carrierconfinement to the light generating region including InAlGaNsemiconductor layer is enhanced. But, the accepter level becomes deeperas the aluminum composition of AlGaN is larger. Consequently, theactivation rate of the p-type dopant is lowered. In the semiconductorlight generating device 1 b, the aluminum composition of the secondAl_(X2)Ga_(1-X2)N semiconductor layer 33 is larger than that of thefirst Al_(X1)Ga_(1-X1)N semiconductor layer 31. P-type dopant atoms ofthe first Al_(X1)Ga_(1-X1)N semiconductor layer 31 can be easilyactivated as compared to the second Al_(X2)Ga_(1-X2)N semiconductorlayer 33. Since the potential barrier of the second Al_(X2)Ga_(1-X2)Nsemiconductor layer 33 to the light generating region 3 b is larger thanthat of the first Al_(X1)Ga_(1-X1)N semiconductor layer 31, these AlGaNsemiconductor layers as a whole confine carriers tightly. In a preferredexample, the thickness of the second Al_(X2)Ga_(1-X2)N semiconductorlayer 33 is not less than five nanometers. If the thickness falls withinthe above range, the light generating region 3 b is sufficientlyseparated from the first Al_(X1)Ga_(1-X1)N semiconductor layer 31 andthe carrier confinement is improved. In a preferred example, thethickness of the second Al_(X2)Ga_(1-X2)N semiconductor layer 33 is notmore than 50 nanometers. This second Al_(X2)Ga_(1-X2)N semiconductorlayer 33 permits the carrier confinement efficiency to be in anacceptable range.

The light generating region 3 b has one or more well layers (forexample, well layers 35 a, 35 b, 35 c) and a plurality of barrier layers(for example, barrier layers 37 a, 37 b, 37 c, 37 d) . The well layers35 a to 35 c and the barrier layers 37 a to 37 d are alternatelyarranged. As shown in area (B) in FIG. 3, the light generating layer 3 bis located between the two AlGaN semiconductor layers, and potentialbarrier greater B1 of the second Al_(X2)Ga_(1-X2)N semiconductor layer33 to the barrier 37 a is greater than the potential barrier B2 of thefirst AlGaN semiconductor layer 32. The third AlGaN semiconductor layer9 provides potential barrier B3 to the barrier 37 d. The barrier layers37 a, 37 b, 37 c, 37 d provide potential barrier B4 to the well layers35 a, 35 b, 35 c.

In a preferred example of the light generating region 3 a and 3 b, theIn_(Y1)Al_(Z1)Ga_(1-Y1-Z1)N well layer satisfies the following:0≦Y1≦0.2; 0≦Z1≦0.5. The In_(Y2)Al_(Z2)Ga_(1-Y2-Z2)N well layer satisfiesthe following: 0≦Y2≦0.2; 0≦Z2≦0.5. The bandgap of theIn_(Y1)Al_(Z1)Ga_(1-Y1-Z1)N is smaller than theIn_(Y2)Al_(Z2)Ga_(1-Y2-Z2)N.

Items of light emitting diode A is listed as a preferred example of thesemiconductor light generating device:

-   -   contact layer 17: Mg doped GaN semiconductor        -   50 nanometers    -   first AlGaN semiconductor layer 5: Mg doped Al_(0.18)Ga_(0.82)N        semiconductor        -   50 nanometers    -   second AlGaN semiconductor layer 33: Mg doped        Al_(0.27)Ga_(0.73)N semiconductor (formed as an undoped        semiconductor layer)        -   20 nanometers    -   light generating region 3 b:    -   well layers 35 a to 35 c: InAlGaN semiconductor        -   15 nanometers    -   barrier layers 37 a to 37 d: InAlGaN semiconductor        -   3 nanometers    -   third AlGaN semiconductor layer 9: Si doped Al_(0.18)Ga_(0.82)N        semiconductor        -   0.2 micrometers    -   buffer layer 15: Si doped GaN semiconductor        -   0.1 micrometers    -   supporting body 13: GaN substrate.

FIG. 4 is a graph showing light generating characteristics of the abovelight emitting diode. The horizontal axis indicates wavelength innanometer, and the vertical axis indicates relative optical powerintensity. The characteristic curve shows an optical spectrum of thelight emitting diode to which 100 mA is continuously fed and the lightgeneration is caused by the band to band transition in InAlGaNsemiconductor. Its peak wavelength is 359 nanometers, which is inultraviolet wavelength region and its half maximum half-width is 16.9nanometers.

FIG. 5 shows a graph showing optical power characteristics of the abovelight emitting diode. The horizontal axis indicates applied current inmA, and the vertical axis indicates optical power in mW. The opticalpower is monotonically increased up to 300 mA as the applied current isincreased, and the optical power is direct proportional to the appliedcurrent.

In order to explain the technical contribution of the second AlGaNsemiconductor layer, light emitting diode B has been prepared. Items oflight emitting diode B is listed as a preferred example of thesemiconductor light generating device:

-   -   contact layer: Mg doped GaN semiconductor        -   50 nanometers    -   first AlGaN semiconductor layer: Mg doped Al_(0.18)Ga_(0.82)N        semiconductor        -   50 nanometers    -   second AlGaN semiconductor layer: Mg doped Al_(0.27)Ga_(0.73)N        semiconductor        -   20 nanometers    -   light generating region:    -   well layers: InAlGaN semiconductor        -   15 nanometers    -   barrier layers: InAlGaN semiconductor        -   3 nanometers    -   third AlGaN semiconductor layer: Si doped Al_(0.18)Ga_(0.82)N        semiconductor        -   0.2 micrometers    -   buffer layer: Si doped GaN semiconductor        -   0.1 micrometers    -   supporting body: GaN substrate.

FIG. 6 shows secondary ion mass spectroscopy (SIMS) analysis of lightemitting diode A. FIG. 7 shows secondary ion mass spectroscopy (SIMS)analysis for light emitting diode B. In FIGS. 6 and 7, lines Al, In, Gaand Mg indicate aluminum, indium, gallium and magnesium dopant profiles,respectively. The horizontal axis in each of FIGS. 6 and 7 indicates adepth from the surface of the light emitting diode. In each figure, thecontact layer, the first AlGaN semiconductor layer, the second AlGaNsemiconductor layer, the light generating region of InAlGaNsemiconductor (containing three well layers and four barrier layers),the third AlGaN semiconductor layer, the buffer layer and GaN substrateare sequentially arranged along the horizontal axis from the origin tothe positive direction. The vertical axis in each figure indicates theconcentrations (or atom counts per second) of the main constituents (Al,In, Ga, Mg).

With reference to FIG. 6, line Mg indicates the following: theconcentration of the magnesium dopant is greater than 1×10²⁰ cm⁻³ in thefirst AlGaN layer and the dopant profile is rapidly decreased outsidethe first and second AlGaN layers, so that the concentration of themagnesium dopant at the interface between the second AlGaN layer and thelight generating region is sufficient low, for example, 1×10¹⁸ cm⁻³.

In FIG. 6, regions S1, S2, S3, S4 and S5 correspond to the contactlayer, the first AlGaN layer, the second AlGaN layer, the lightgenerating region and the third AlGaN layer, respectively.

With reference to FIG. 7, line Mg indicates the following: theconcentration of the magnesium dopant is greater than 1×10¹⁹ cm⁻³ in thefirst AlGaN layer and the dopant profile is gentle decline in the secondAlGaN layer, so that the concentration of the magnesium dopant at theinterface between the second AlGaN layer and the light generating regionis not sufficiently low, for example, 3×10¹⁸ cm⁻³. The concentration ofthe magnesium dopant is further decreased in the light generating regionand finally becomes sufficiently low value (10¹⁷ cm⁻³).

In FIG. 7, regions T1, T2, T3, T4 and T5 correspond to the contactlayer, the first AlGaN layer, the second AlGaN layer, the lightgenerating region and the third AlGaN layer, respectively.

FIG. 8 is a graph showing the characteristics of the above lightemitting diodes. Line C_(A) indicates the characteristics of lightemitting diode A and Line C_(B) indicates the characteristics of lightemitting diode B. This graph reveals that the optical power of lightemitting diode A is twice as large as that of light emitting diode Btwice in the range up to 300 milliamperes.

The second AlGaN semiconductor layer in light emitting diode B is dopedwith Magnesium (Mg) and the second AlGaN semiconductor layer in lightemitting diode A is undoped. The difference between the curves in FIG. 8comes from the undoped second AlGaN semiconductor layer.

FIG. 9 is a graph showing optical outputs of three light emittingdiodes. Symbols C18, C24 and C27 indicate the characteristic curves ofaluminum compositions (X), 0.18, 0.24 and 0.27 of the secondAl_(X)Ga_(1-X)N layer, respectively. As shown in FIG. 9, the opticalpower is enhanced as the aluminum composition of the second AlGaN layeris large. The optical power is maximized at the composition of 27percent and then is decreased in the aluminum composition over 27percent.

Although the p-type dopant profile of the second AlGaN layer and thelight generating region in the modified semiconductor light generatingdevice 1 b may be the same or similar to that shown in area (B) in FIG.2, the p-type dopant profile in the modified semiconductor lightgenerating device 1 b is not limited thereto.

It is preferable that the aluminum composition of the Al_(X)Ga_(1-X)Ncladding layer for the light generating region including one or moreInAlGaN layers is greater than 0.1 (0.1<X), and more preferably, thealuminum composition is less than or equal to 0.3 (X≦0.3).

As explained above, the light generating semiconductor device includingInAlGaN semiconductor according to the present embodiment improves thelight generating efficiency thereof.

Second Embodiment

Areas (A) and (B) in FIGS. 10 and areas (A) and (B) in FIG. 11 showsteps of fabricating the light generating semiconductor device accordingto the present embodiment.

As shown in area (A) in FIG. 10, a substrate 41, such as n-type GaNsemiconductor single crystal substrate is prepared. Then, a number ofsemiconductor films of III group nitride are formed on the substrateusing a metal-organic vapor phase epitaxy apparatus.

As shown in area (A) in FIG. 10, the substrate 41 is mounted on thesusceptor 43 a in the metal-organic vapor phase epitaxy apparatus 43.After adjusting the growth temperature, raw material gases are suppliedto the evacuated vessel in the metal-organic vapor phase epitaxyapparatus 43 to form a film on the substrate 41. For example, thefollowing raw material gases are used: trimethyl-gallium as a galliumsource, trimethyl-aluminum as an aluminum source, trimethyl-indiumadduct as an indium source, ammonia as a nitrogen source, tetraethylsilane as a silicon source, and bisethyl-cyclopentadienyl magnesium as amagnesium source.

The growth temperature is set at 1050 Celsius degrees. Tetraethylsilane, trimethyl-gallium and ammonia gases are supplied to the vesselto form an n-type GaN film 45 on the substrate 41. The GaN film 45 isused as a buffer layer. The thickness is, for example, 0.1 micrometersand the dopant concentration is, for example, about 2×10¹⁸ cm⁻³.

The temperature is kept unchanged and trimethyl-aluminum, tetraethylsilane, trimethyl-gallium and ammonia gases are supplied to the vesselto form an n-type AlGaN film 47 on the GaN film 45. The n-type AlGaNfilm 47 is used as a hole block layer. The thickness is, for example,0.2 micrometers, its composition is, for example, Al_(0.18)Ga_(0.82)Nand the dopant concentration is, for example, about 2×10¹⁸ cm⁻³.

As shown in area (B) in FIG. 10, a light generating region 49 is thenformed on the AlGaN film 47. The light generating region 49 includes anundoped InAlGaN semiconductor film. The growth temperature is set at 830Celsius degrees. Trimethyl-aluminum, trimethyl-indium adduct,trimethyl-gallium and ammonia gases are supplied to the vessel to forman In_(0.05)Al_(0.25)Ga_(0.70)N film on the n-type AlGaN film 47. Thecomposition of this film is determined using Rutherford Back ScatteringMethod. This InAlGaN film is used as a barrier layer and the thicknessis, for example, 15 nanometers.

Then, trimethyl-aluminum, trimethyl-indium adduct, trimethyl-gallium andammonia gases are supplied to the vessel to form anIn_(0.05)Al_(0.25)Ga_(0.75)N film on the n-type AlGaN film 47. ThisInAlGaN film is used as a well layer and the thickness is, for example,3 nanometers. Subsequently, trimethyl-aluminum, trimethyl-indium adduct,trimethyl-gallium and ammonia gases are supplied to the vessel to forman In_(0.05)Al_(0.25)Ga_(0.70)N film on the n-type AlGaN film 47. TheInAlGaN film is used as a barrier layer and the thickness is, forexample, 15 nanometers. The well layer and the barrier layer isrepeatedly deposited until desired light generating region is formed,for example, three times. The repetition forms the light generatingregion having a multiple quantum well structure.

One example of an InAlGaN barrier layer is:

-   -   ammonia: 2 litters per minute    -   trimethyl-gallium: 1.5 micromoles per minute    -   trimethyl-aluminum: 0.65 micromoles per minute    -   trimethyl-indium adduct: 30 micromoles per minute.

One example of an InAlGaN well layer is:

-   -   ammonia: 2 litters per minute    -   trimethyl-gallium: 1.5 micromoles per minute    -   trimethyl-aluminum: 0.52 micromoles per minute    -   trimethyl-indium adduct: 53 micromoles per minute.

In area (A) in FIG. 11, the growth temperature is set at 1050 Celsiusdegrees. An undoped Al_(X2)Ga_(1-X2)N semiconductor film (0≦X2≦1) isformed on the light generating region 49 including InAlGaN semiconductorfilm. For example, trimethyl-gallium, trimethyl-aluminum and ammoniagases are supplied to the vessel to form an undoped AlGaN film 51 on thelight generating region 49. This AlGaN film 51 is used as an electronblock layer. Its thickness is, for example, 20 nanometers and itscomposition is, for example, Al_(0.27)Ga_(0.73)N.

Subsequently, the temperature is kept unchanged and a p-typeAl_(X1)Ga_(1-X1)N semiconductor (0≦X1≦1) film 53 and one or moreGaN-based semiconductor film, such as a contact film 55, are formed onthe undoped AlGaN film 51. During the deposition of these films, p-typedopant atoms in the Al_(X1)Ga_(1-X1)N semiconductor film 53 diffuse intothe undoped AlGaN film 51, and the undoped AlGaN film 51 reduces thenumber of p-type dopant atoms diffusing from the Al_(X1)Ga_(1-X1)Nsemiconductor film 53 to the light generating region 49. Most part ofthe light generating region 49 does not contain the p-type dopant atomsfrom the Al_(X1)Ga_(1-X1)N semiconductor film 53 and thus issubstantially undoped.

Trimethyl-aluminum, bisethyl-cyclopentadienyl-magnesiumtrimethyl-gallium and ammonia gases are supplied to the vessel to form ap-type AlGaN film 53 on the undoped AlGaN film 51, the p-type AlGaN film53 is used as an electron block layer. Its thickness is, for example, 50nanometers, its composition is, for example, Al_(0.18)Ga_(0.82)N, andits dopant concentration is, for example, about 2×10²⁰ cm⁻³.

In area (B) in FIG. 11, bisethyl-cyclopentadienyl magnesium,trimethyl-gallium, ammonia is supplied to deposit a p-type GaN layer isformed on the p-type GaN layer 55 on the p-type AlGaN semiconductorlayer 53. The p-type GaN film 55 works as a p-type contact layer, itsthickness is, for example, 50 nanometers, and its dopant concentrationis about 2×10²⁰ cm⁻³.

After the above steps, a number of GaN-based semiconductor films aredeposited on the substrate to form a substrate production. Then, asemi-transparent electrode (anode electrode) is formed on the contactfilm 55 and another electrode (cathode electrode) is formed on thebackside of the substrate 41.

As explained above, a method of fabricating a semiconductor lightgenerating device including InAlGaN semiconductor is provided.

Since the growth temperatures for AlGaN semiconductor and GaNsemiconductor is higher than the growth temperature for InAlGaNsemiconductor, it is not likely to form pits on AlGaN semiconductor andGaN semiconductor but it is likely to form more pits on InAlGaNsemiconductor as compared to AlGaN semiconductor and GaN semiconductor.InAlGaN semiconductor containing many pits facilitates to diffuse p-typedopant magnesium atoms therein and thus the light generating efficiencyis deteriorated. In the semiconductor light generating device asexplained above, the effect of the p-type dopant magnesium atoms isreduced. Since the temperature favorable for growing InAlGaNsemiconductor is lower than temperatures for AlGaN semiconductor and GaNsemiconductor, it is likely to form more pits on InAlGaN semiconductoras compared to AlGaN semiconductor and GaN semiconductor. By reducingthe quantity of p-type dopant magnesium atoms in the InAlGaN layers andby doping the AlGaN layer adjacent to the InAlGaN layers with p-typedopant magnesium atoms by diffusion to lower the resistance of thesemiconductor light generating device, the semiconductor lightgenerating device including InAlGaN semiconductor layers prevents thelight generating efficiency from decreasing.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. For example, the presentembodiments have explained the semiconductor surface emitting devices,such as light emitting diodes, but those having skill in the artappreciates the present invention can be used for laser diodes. Thepresent invention is not limited to the specific examples disclosed inthe specification. Details of structures of these devices can bemodified as necessary. Further, sapphire substrates can be used as asupporting body. We therefore claim all modifications and variationscoming within the spirit and scope of the following claims.

1. A semiconductor light generating device comprising, a lightgenerating region including a InAlGaN semiconductor layer; a firstAl_(X1)Ga_(1-X1)N semiconductor layer (0≦X1≦1) doped with p-type dopant;and a second Al_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1) having ap-type dopant concentration smaller than that of the firstAl_(X1)Ga_(1-X1)N semiconductor layer, the second Al_(X2)Ga_(1-X2)Nsemiconductor layer being provided between the InAlGaN semiconductorlayer and the first Al_(X1)Ga_(1-X1)N semiconductor layer.
 2. Thesemiconductor light generating device according to claim 1, whereinthickness of the second Al_(X2)Ga_(1-X2)N semiconductor layer is greaterthan or equal to five nanometers.
 3. The semiconductor light generatingdevice according to claim 1, wherein the first Al_(X1)Ga_(1-X1)Nsemiconductor layer is doped with magnesium of p-type dopant, andwherein the first Al_(X1)Ga_(1-X1)N semiconductor layer includes aregion having a magnesium concentration of 1×10¹⁹ cm⁻³ or higher.
 4. Thesemiconductor light generating device according to claim 1, wherein thesecond Al_(X2)Ga_(1-X2)N semiconductor layer is doped with magnesiumdopant, and wherein a p-type dopant concentration at an interfacebetween the light generating region and the second Al_(X2)Ga_(1-X2)Nsemiconductor layer is less than 3×10¹⁸ cm⁻³ or lower.
 5. Thesemiconductor light generating device according to claim 1, wherein amaximum concentration of p-type dopant in the first Al_(X1)Ga_(1-X1)Nsemiconductor layer is not less than 1×10²⁰ cm⁻³, wherein a p-typedopant concentration at an interface between the light generating regionand the second Al_(X2)Ga_(1-X2)N semiconductor layer is not greater than3×10¹⁸ cm⁻³.
 6. The semiconductor light generating device according toclaim 1, further comprising: a supporting body of III-group nitride, thelight generating region, the first Al_(X1)Ga_(1-X1)N semiconductor layerand the second Al_(X2)Ga_(1-X2)N semiconductor layer being provided onthe supporting body.
 7. The semiconductor light generating deviceaccording to claim 1, wherein the second Al_(X2)Ga_(1-X2)N semiconductorlayer is formed as an undoped layer.
 8. The semiconductor lightgenerating device according to claim 1, wherein the secondAl_(X2)Ga_(1-X2)N semiconductor layer (0≦X2≦1) has an aluminumcomposition greater than that of the first Al_(X1)Ga_(1-X1)Nsemiconductor layer.
 9. The semiconductor light generating deviceaccording to claim 1, wherein the light generating region furtherincludes one or more InAlGaN semiconductor barrier layers and one ormore InAlGaN semiconductor well layers.
 10. A semiconductor lightgenerating device comprising; a light generating region including aInAlGaN semiconductor layer; a first Al_(X1)Ga_(1-X1)N semiconductorlayer (0≦X1≦1) doped with p-type dopant; and a second Al_(X2)Ga_(1-X2)Nsemiconductor layer (0≦X2≦1) having an aluminum composition greater thanthat of the first Al_(X1)Ga_(1-X1)N semiconductor layer, the secondAl_(X2)Ga_(1-X2)N semiconductor layer being provided between the InAlGaNsemiconductor layer and the first Al_(X1)Ga_(1-X1)N semiconductor layer,and the p-type dopant concentration of the second Al_(X2)Ga_(1-X2)Nsemiconductor layer being smaller than that of the firstAl_(X1)Ga_(1-X1)N semiconductor layer.
 11. The semiconductor lightgenerating device according to claim 10, wherein the secondAl_(X2)Ga_(1-X2)N semiconductor layer includes a region having a p-typeconcentration of 3×10¹⁸ cm⁻³ or lower, and wherein this region is 1nanometer or more.
 12. The semiconductor light generating deviceaccording to claim 10, wherein the first Al_(X1)Ga_(1-X1)N semiconductorlayer is doped with magnesium of p-type dopant, and wherein the firstAl_(X1)Ga_(1-X1)N semiconductor layer includes a region having amagnesium concentration of 1×10¹⁹ cm⁻³ or higher.
 13. The semiconductorlight generating device according to claim 10, wherein the secondAl_(X2)Ga_(1-X2)N semiconductor layer is doped with magnesium dopant,and wherein a p-type dopant concentration at an interface between thelight generating region and the second Al_(X2)Ga_(1-X2)N semiconductorlayer is less than 3×10¹⁸ cm⁻³ or lower.
 14. The semiconductor lightgenerating device according to claim 10, wherein a maximum concentrationof p-type dopant in the first Al_(X1)Ga_(1-X1)N semiconductor layer isnot less than 1×10²⁰ cm⁻³, wherein a p-type dopant concentration at aninterface between the light generating region and the secondAl_(X2)Ga_(1-X2)N semiconductor layer is not greater than 3×10¹⁸ cm⁻³.15. The semiconductor light generating device according to claim 10,further comprising: a supporting body of III-group nitride, the lightgenerating region, the first Al_(X1)Ga_(1-X1)N semiconductor layer andthe second Al_(X2)Ga_(1-X2)N semiconductor layer being provided on thesupporting body.
 16. The semiconductor light generating device accordingto claim 10, wherein the second Al_(X2)Ga_(1-X2)N semiconductor layer isformed as an undoped layer.
 17. The semiconductor light generatingdevice according to claim 10, wherein the light generating regionfurther includes one or more InAlGaN semiconductor barrier layers andone or more InAlGaN semiconductor well layers.
 18. A method of making asemiconductor light generating device according to claim 1, the methodcomprising the steps of: forming an undoped InAlGaN semiconductor filmfor an active region; forming an undoped Al_(X2)Ga_(1-X2)N semiconductorfilm (0≦X2≦1) on the InAlGaN semiconductor film; and forming a p-typeAl_(X1)Ga_(1-X1)N semiconductor film (0≦X1≦1) and one or more galliumnitride based semiconductor films on the undoped Al_(X2)Ga_(1-X2)Nsemiconductor film and causing p-type dopant atoms in the p-typeAl_(X1)Ga_(1-X1)N semiconductor film to diffuse to the undopedAl_(X2)Ga_(1-X2)N semiconductor film.
 19. A method of making asemiconductor light generating device according to claim 9, the methodcomprising the steps of: forming an undoped InAlGaN semiconductor filmfor an active region; forming an undoped Al_(X2)Ga_(1-X2)N semiconductorfilm (0≦X2≦1) on the InAlGaN semiconductor film; and forming a p-typeAl_(X1)Ga_(1-X1)N semiconductor film (0≦X1≦1) and one or more galliumnitride based semiconductor films on the undoped Al_(X2)Ga_(1-X2)Nsemiconductor film and causing p-type dopant atoms in the p-typeAl_(X1)Ga_(1-X1)N semiconductor film to diffuse to the undopedAl_(X2)Ga_(1-X2)N semiconductor film.
 20. The method according to claim19, wherein aluminum composition of the Al_(X2)Ga_(1-X2)N semiconductorfilm is greater than that of the p-type Al_(X1)Ga_(1-X1)N semiconductorfilm (0≦X1≦1).